Forklift trucks have been used either to lift objects of relatively heavy weight up to an elevated location or to move the objects from one place to another within a limited working site. Depending on the power sources employed, the forklift trucks are classified into an engine-driven forklift truck which may usually operate in an outdoor area and an electromotive forklift truck which are suitable for indoor operation, thanks to its reduced or little emission of exhaust gas and noise.
It is well known in the art that the electromotive forklift trucks carry out the steering operation and the tilting and lifting operation through the use of a pressurized working fluid discharged from a hydraulic pump which in turn is rotatingly driven by an electric pump motor. The working fluid is fed to a priority valve that serves to supply the working fluid preferentially to a steering unit to thereby assure stable steering operation at all times. The surplus fluid is directed to such an working implement as a lift cylinder and a tilt cylinder by way of a control valve unit.
The discharge volume of the hydraulic pump depends primarily on the torque of the electric pump motor, the torque and speed of which is controlled by means of a pump motor controller. An example of the prior art pump motor controller is shown in FIG. 1 wherein a battery 10 is employed to feed electricity to the respective one of electric devices of the forklift truck. The pump motor controller further includes a key switch 11 for producing a key input signal to start up the forklift truck and a mast manipulation switch 12 for effecting the tilting and lifting operation of the working implement. A valve control unit 13 functions to control hydraulic valves for selectively supplying the working implement with the pressurized fluid in response to the switching signals from the mast manipulation switch 12. Responsive to the valve control signals from the valve control unit 13, a controller 14 is adapted to produce pulse width modulation signals of a predetermined on-duty ratio which can be used to control a pump motor 15. A driving transistor 16 responds to the pulse width modulation signals to enable the battery 10 to supply electricity to the pump motor 15, while a flywheel diode 17 has a function to assure continuous flow of an electric current through the pump motor 15. The cathode of the flywheel diode 17 is connected to between the emitter of the driving transistor 16 and the pump motor 15, with the anode thereof being grounded.
The pump motor controller of the construction as noted above is operable in the following manner. As the driver turns on the key switch 11 to start up the forklift truck, the electric current is fed from the battery 10 to a travel motor (not shown) and the pump motor 15, thereby energizing them at the same moment. Pressing the mast manipulation switch 12 for the sake of performing the tilting and lifting operation causes the valve control unit 13 to properly shift the control valves (not shown), whereby switching signals indicative of the valve actuation is applied to the controller 14. Concurrently fed to the controller 14 are steering signals which represent the steering condition in the travelling process of the forklift truck.
In response to the switching signals and the steering signals, the controller 14 supplies the driving transistor 16 with pulse width modulation signals of a predetermined on-off duty ratio, which causes the driving transistor 16 to be switched such that an electric current corresponding to the pulse width modulation signals can flow from the collector to the emitter of the driving transistor 16 to thus energize the pump motor 15. At this moment, the flywheel diode 17 makes smooth the negative electromotive forces developed at the opposite ends of the pump motor 15 in the event of shut-down of the driving transistor 16, assuring a reduced consumption of the battery electricity and a suppressed over-voltage owing to the inductive resistance of the pump motor 15.
According to the prior art pump motor controller as referred to above, the on-off duty ratio of the pulse width modulation signals is predetermined, meaning that the torque and speed of the pump motor is controlled depending solely on the on-off duty ratio of the pulse width modulation signals but regardless of the load variation exerting on the pump motor. This makes it impossible to precisely control the torque of the pump motor in close response to the load variation. Moreover, controlling the pump motor in this manner tends to create irregular noise in the pump motor and fluid lines especially under the unloaded condition, adversely affecting the convenience of forklift operation.